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How to point patio slabs with a damp mortar

Updated: October 2026

You are about to fill joints on concrete flags that already sit still. Two mistakes waste the afternoon: a dry brush across the face, and a whole bag of rapid mortar. This is how to point patio slabs with a damp mix in small batches. Cards are on the [paving joint mortar comparison](/hub/flooring-tools-and-accessories/paving-joint-mortar). Which bag to buy is [best paving joint mortar](/hub/flooring-tools-and-accessories/paving-joint-mortar/sub/paving-joint-mortar-best). Rigid joint versus brush-in tub is [jointing compound vs sand and cement](/hub/flooring-tools-and-accessories/paving-joint-mortar/sub/paving-joint-mortar-vs-sand-and-cement). If the units are 20–30 mm porcelain, read [porcelain paving joint mortar](/hub/flooring-tools-and-accessories/paving-joint-mortar/sub/paving-joint-mortar-porcelain) instead of copying this method.

You are about to fill joints on concrete flags that already sit still. Two mistakes waste the afternoon: a dry brush across the face, and a whole bag of rapid mortar.

Our top picks

Bostik Bostik Cementone mortar, 10 kg

Best overall

Bostik Bostik Cementone mortar, 10 kg
9.0/10about £14.99
View offer
Sika Everbuild Everbuild Jetcem rapid repair, 6 kg

Alternative

Sika Everbuild Everbuild Jetcem rapid repair, 6 kg
8.8/10about £10.70
View offer
Siroflex SX Siroflex mortar repair, 300 ml

Best value

Siroflex SX Siroflex mortar repair, 300 ml
8.4/10about £7.96
View offer
On this page
  1. 1.Check the slabs before you mix
  2. 2.Mix a damp mortar, not a dry brush
  3. 3.Small batches for a rapid premix or a cartridge
  4. 4.Dust, wet cement, and when to stop

Check the slabs before you mix

Stand on the corners before you open a bag. A flag that rocks, or a hollow spot that sounds different underfoot, is not ready to point. Re-bed that slab on a full mortar bed first. Marshalls' concrete paving guidance assumes that full bed. If old mortar still fills the joint, you need a clean channel, not a smear over the top. A joint around 10 mm on a flag at least about 22 mm thick is the sort of gap this method is written for. A 2 mm line is a different product.

Mix a damp mortar, not a dry brush

Mix the powders dry first so the colour is even, then add water until the mortar holds a shape and is not a slurry. Press it in from a hawk or a pointing bar. Sponge the face while the cement is still wet. A faint smear becomes a grey bloom once it dries. Work a short length, tool the joint, wipe, then move.
Bostik Bostik Cementone mortar, 10 kg

Testsieger

Bostik Bostik Cementone mortar, 10 kg
9.0/10about £14.99
View offer

Small batches for a rapid premix or a cartridge

Do not pour water into a Jetcem batch that has already stiffened. Mix a little, place it, then mix again. Cementone wants very little water, and a small mix, so the rest of the 10 kg tub stays powder. The grey dries light if you leave a film on the slab. The SX cartridge is not a damp mortar. If the joint is still dark from last night's rain, leave the gun in the box.

Dust, wet cement, and when to stop

Raking old mortar throws up respirable silica. Wet the work where you can. Do not dry-grind a large area because it feels quicker. Wet cement can burn. Wear gloves and wash splashes off rather than waiting until the pointing is finished. A chromium VI reducer only lasts until the shelf date on the bag. A stable garden path, a damp mix in small batches and a sponge on the face is the job this page is for.

Common questions

Do you brush dry sand and cement into the joints?
No. Marshalls' concrete paving guidelines say dry or semi-dry sand and cement must not be brushed in. The dust stains the face, and you do not get the dense damp joint the same sheet describes. Mix the powders first, add water until the mortar holds, press it in, and sponge the slab straight away. A blog that still shows a dry brush is the method to refuse.
What mix does Marshalls describe for ordinary flags?
For many concrete flags the sheet describes about 1 part cement to 4 parts building sand, in joints around 10 mm wide, on flags at least about 22 mm thick, over a full bed. The Home Pack listings at 1:6 are a leaner ratio, and reviews of those bags include soft joints. Use 1:4 when you are mixing for this sort of flag. Do not rename a 1:6 bag as the same point.
How much Jetcem should you mix?
Mix a small batch of the 6 kg premix, not the bag. The listing full-cure field is 30 minutes, and reviews often say the mix is unusable in about 10. Once a batch has stiffened, start a fresh one rather than drowning it. That pace suits cracks, steps and a few slab joints. It is a poor way to point a whole terrace from one mix.
When can you use the SX cartridge?
Use the 300 ml tube when the repair is small, the weather is dry, and both the product and the slab are above 5°C. Keep the surface dry for at least five hours. You need a separate gun. Reviewers find it stiff and short on coverage, and it does not willingly stick to a damp joint. It is not the method for a patio you still have to point in the wet.
What protection do you need when raking old mortar?
Treat the dust as silica. UK guidance sets a workplace exposure limit of 0.1 mg/m³ over eight hours and expects extraction plus respiratory protection at APF 20 for raking and chasing. Gloves matter because wet cement burns. A large dry grind is where a domestic mask and a weekend are the wrong tools.
What if the slabs move when you stand on them?
Stop pointing. Movement means the bed has failed, and a new joint will crack with it. Lift the loose flags, re-bed them fully, and only then point. A hollow sound under an otherwise tidy slab is the same stop. No mix in this set turns a rocking flag into a stable one.
When should you stop and call a professional?
Call someone when the slabs are hollow or rocking, the fall is towards the house, vehicles will use the area, the building is listed or in a conservation area, the frontage crosses the hard-surface rule where you live, or clearing the joints means a long dry grind. A stable path with open joints is the DIY point. The other cases are re-bedding, drainage or consent.

Related pages

Paving joint mortar buying guide

Paving joint mortar

Best paving joint mortar for the job in front of you

Porcelain paving joint mortar for thin 20–30 mm slabs

Jointing compound vs sand and cement on a single patio

Sources

Public sources used for this comparison, with the amazon.co.uk listings read on 20 October 2026.

  1. Bostik UK TDS02249, All Weather jointing compound (retrieved 20 October 2026)
  2. Joint-it Simple (retrieved 20 October 2026)
  3. Dansand Kerama (retrieved 20 October 2026)
  4. Marshalls concrete paving installation guidelines (retrieved 20 October 2026)
  5. England: permitted development technical guidance (retrieved 20 October 2026)
  6. BSI on BS 7533-101 (retrieved 20 October 2026)
  7. HSE: cement (retrieved 20 October 2026)
  8. HSE: chasing concrete and raking mortar (retrieved 20 October 2026)